1. Field of the Invention
The present invention concerns a process and method for wide band transmission particularly for data transmission over an electricity distribution network, in particular a low voltage network.
2. Description of the Prior Art
A particular but not exclusive area of application of the invention is that of automatic transmission to a central point of meter readings of electricity or other services assigned to several respective flats or dwellings, in a way to allow a remote meter reading.
On top of the obvious advantages which obtain from the automatic centralization of meter readings, the use of the network as a transmission path allows special transmission links between the central point and the meters to be dispersed with.
However, by reason of the uncertainty and distortion of its transmission characteristics and the presence of noise, the network is a difficult path to use for reliable data transmission.
In fact, examination of distribution network characteristics shows variations in attenuation as a function of frequency, with pronounced dips in transmission at some frequencies, such variations being different between one network and another.
Noise is of several types. Some is a fixed frequency signal resulting from the switching of inductive loads; this is impulse noise the amplitude of which may be relatively large. A second source of noise arises at harmonics of the frequency of the network, but these are sufficiently strongly attenuated above a few kHz.
There is also gaussian background noise the amplitude of which, by way of example, may be of the order of 1mV over a frequency band of 50 Hz centered at 100 kHz. There may also be fluctuating signals injected, such as those caused by televisions and intercoms.
To improve transmission security under such conditions wide band transmission has been proposed. Such a transmission process is described in U.S. Pat. No. 4,307,380.
More generally, it is known to use specific data coding and decoding methods to provide wide band transmission with a view toward increasing transmission security and deciphering immunity.